1. Field of the Invention
The present invention relates to a binder for dry toners used for developing electrostatic latent images formed by electrophotography.
2. Description of the Prior Art
Up to now, there have been known a variety of electrophotographic methods such as those disclosed in U.S. Pat. No. 2,297,691 and Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") No. Sho 43-24748. In general, the electrophotographic method comprises forming various electrostatic latent images on a photoconductive element which is composed of a photoconductive material such as selenium, zinc oxide or cadmium sulfide according to various methods, electrically adhering toner particles to the latent images to give toner images and then transferring the toner images to a substrate such as paper to thus give a copy.
The heat fusing process by means of the heating roller has become the leading fixing method in electrophotography from the viewpoint of speeding up of the copying operations and of energy-saving. Moreover, it is necessary that the toner must be electrostatically electrified at a polarity falling within an optimum range through frictional contact thereof with a carrier in order to obtain clear images. For this reason, it has been desired to develop a resin as a binder for the toner which can withstand high speed copying operation and can provide copies of high quality.
Styrene-acrylic resins have been most widely used as the toner binders. These resins are cheap and have excellent resistance to humidity as well as high resistance to blocking, i.e., resistance to the phenomenon that toner particles are adhered to one another during the storage thereof or the so-called blocking phenomenon, but these resins have, on the contrary, low mechanical strength and low rate of electrification.
It has been known that polyester resins are excellent in mechanical strength and have a high rate of electrification, but have low resistance to humidity which in turn leads to lowering of the electrifying properties when humidity is high and the resins are relatively expensive.
Under such circumstances, there have been proposed many attempts for improving the properties of toner binders by coupling a styrene-acrylic resin with a polyester resin. For instance, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 63-127245 discloses a method which comprises melting and kneading a styrene-acrylic copolymer and a polyester resin in a twin-roll mill to react them. Moreover, J.P. KOKAI No. Sho 63-27855 discloses a method for preparing a resin for toner binders by reacting a crystalline polyester resin with an amorphous vinyl polymer Both of these patents utilize a polymer/polymer reaction. In general, the reaction rate of such polymer/polymer reactions is low and the end point of these reactions is also unclear. Thus, these methods suffer from problems in that the productivity is low and that the quality control of the resulting products is difficult.
Moreover, J.P. KOKAI No. Sho 59-45453 discloses a method for preparing a resin used as toner binders which comprises subjecting a polyester resin carrying terminal hydroxyl groups and (meth)acrylic acid to ester-condensation to form a polyester resin having at least one (meth)acryloyl group at the end of the molecule, dissolving the resulting polyester resin in a vinyl compound monomer and then polymerizing them. However, in this method, the rate of the esterification is low and the linkage between the polyester resin and the vinyl compound monomer is insufficient. Therefore, the resulting product is not acceptable as a toner binder.